Couplings conventionally are used to transfer motion from one machine shaft to another. Numerous issues are presented in designing a suitable coupling, with variables such as torque, speed, misalignment, dampening, backlash, cost, size, reliability, ease of maintenance, and ease of manufacture needing to be considered. Consequently, many couplings either are designed for particular purposes or function optimally only in certain circumstances.
At least some couplings are rigid, providing essentially no flexibility. Often cylindrical in shape, these couplings include openings on each end of the cylinder to receive shafts. Typically set screws are used to lock the received shafts into position, although some versions of rigid couplers employ alternative fasteners (such as square keys or woodruff keys). Regardless of fastening mechanism, though, these couplings tolerate essentially no shaft misalignment and cannot provide any dampening.
Other existing couplings are known as “spider” couplings. Also generally cylindrically shaped, these couplings include protruding members and utilize a cushion (the “spider”) between cylindrical hubs. Although the machine shafts are locked in position relative to each hub (as they are in rigid couplings), employing a spider made of elastomeric material accommodates at least modest shaft misalignment and allows, potentially, for some dampening to occur. By contrast, possible failures of the protruding members under load or shock limit the functionality of these couplings.
Yet other couplings presently available are called helical beam couplings. Formed, basically, of tubes with helixes cut around the tube walls, these couplings nevertheless maintain rigid hub-shaft connections at their ends. Consequently, although they sometimes may admit some parallel misalignment, they typically permit no axial misalignment of shafts or dampening of the motion. Helical beam couplings further likely will not support high torque levels and are relatively expensive.
Another type of commercially-available coupling is the slotted-disc, or Oldham, coupling. In this device a hard slotted disc replaces the spider of the spider coupling, allowing the ends of the coupling to move independently of one another. Failures of the disc may occur, however, and no axial (and little angular) misalignment of shafts is permitted. Oldham couplings additionally permit only small amounts of dampening.
Pinhole-disc couplings (also known as Schmidt couplings) likewise are similar to spider couplings. However, rather than utilizing protruding members, pinhole-disc couplings employ dowel pins on half of the face of the coupling hub. Hubs are joined with special flexible discs that clock them at ninety degrees to each other. Again, though the (usually plastic) discs, or the dowel pins, may fail in use; additionally, no axial shaft misalignment is permitted and only limited dampening is available.
Bellows and gear-and-sleeve couplings provide further alternatives to the couplings heretofore described. Bellows couplings, with their accordion-style shapes, are highly flexible. They permit neither axial shaft misalignment nor dampening to occur, however, and because of their shape are relatively expensive to manufacture. Gear-and-sleeve couplings, by contrast, allow some dampening. Typically consisting of metallic hubs with external gear teeth that slide into tubular sleeves with complementary grooved teeth, these couplings are advantageous when significant axial misalignment is expected. However, they are relatively expensive to manufacture, require substantial maintenance, tend to vibrate at high speeds, and need lubrication on many metal sleeve designs.
Among other conventional couplings are double-loop ones, comprising two hubs with a flexible double loop of elastomeric material molded so as to provide an offset figure-eight to each hub. These couplings fail to permit dampening and are relatively expensive to manufacture; as well, because the elastomeric material is large, they require substantial space for operation. Tire couplings likewise comprise hubs (albeit large metallic ones) connected by an elastomeric “tire.” Tire walls are clamped to the large hubs while smaller quick-disconnect bushings lock the hubs to the shafts on each end. Similar to the double-loop couplings, these tire couplings require significant space in which to operate and are expensive to produce. They further are heavier than most other couplings and do not support large torques.
Shear couplings attempt to protect over-driven shafts from damage. These couplings include two metallic cylindrical hubs, the ends of which receive the shafts, and a molded elastomeric member between them. Rather than supporting high-torque operation, the member contains a center section designed to fail when subjected to high torque so as to reduce the risk of the shaft doing so. Shear couplings also do not permit any axial misalignment of shafts and allow only low angular and parallel misalignments.
Multi-flex couplings, like many others, include two metallic hubs with a central elastomeric element. Each hub has a groove on its face that contains teeth, and the elastomeric element has integrally-formed teeth on its inner and outer sides at each of its ends. The teeth of the elastomeric element fit into each hub. Possibility of failure of the elastomeric elements remains an issue for these couplings, are does their limited ability to tolerate misaligned shafts.
Roller-chain couplings provide yet other alternative devices. Consisting of hubs with external gear teeth on an end, these couplings are joined y a roller chain set into the gear teeth so as to lock the hubs together. A cover wraps each set of hubs and chain to complete an assembly. Lubrication is required for the couplings, however, and misalignment tolerances are small. Failure of the chain, further, will result in the coupling being unable to transfer rotational motion.
Frontline Industries, Inc. of Irvington, N.J. advertises yet another coupling under the name “Big Boy.” This coupling consists of a hub with a center hole to mount shafts and multiple bores positioned around the hub face. A center ring, containing twice as many holes as the number of hub bores, accepts threaded, bullet-shaped pins. Each assembly includes a hub, pins installed on each side of the hub, and a cylindrical rubber bushing placed over the pins, which are then inserted into the bores. Among disadvantages of the “Big Boy” coupling are that it appears to require close tolerances for operation and is expensive to manufacture. The coupling also could disengage if axial misalignment exists above a modest level.
Finally, also advertised as commercially available is the “Superflex Super Elastic Coupling.” This coupling incorporates a flexible center section connected to two metallic hubs with through bolts positioned in an alternating pattern. It is large in size, however, and both expensive and designed for heavy industrial use.